1. Field of the Invention
The present invention relates to a superconducting motor utilizing bulk-shaped superconductors.
2. Description of the Related Art
The device disclosed in Japanese unexamined patent publication No. 289344/1991 has been known as a superconducting motor.
This superconducting motor is comprised of a shaft, a disk-shaped armature secured to the shaft and superconducting coils disposed opposite to the armature. The superconducting coils also have a container for liquid helium and a radiation shield.
This superconducting motor is of a type wherein a magnetic field generated from the excited superconducting coil provides the armature with a torque.
Specifically, the superconducting coils have been cooled by liquid helium below the critical temperature thereof and are in a superconducting state. Therefore, the superconducting coils are easily excited by supplying an electric current thereto so that they are turned into magnets having a high magnetic flux density.
However, the above-described superconducting motor has problems as follows. First, quenching may occur at the superconducting coils. The term "quenching" means a phenomenon that a normal conducting area which has been locally generated expands continuously causing the entire superconductor to be rapidly transformed into a normal conducting state. Such quenching results from applying a current and/or a magnetic field higher than a critical current and/or a critical magnetic field specific to the superconductor. Further, such quenching is likely to occur especially in wires.
In addition, a superconductor has an electric resistance in the normal conducting state. Therefore, Joule heat is generated corresponding to an electric current flowing therein.
Therefore, when quenching occurs in the superconducting coils of the above-described superconducting motor, the Joule heat generated from the coils can cause the liquid helium to boil, resulting in damages in the liquid helium container etc. due to an increase in the pressure therein. The Joule heat can also melt down the coils themselves.
Accordingly, the output of the conventional superconducting motor as described above is limited because attention must be paid to quenching. This also adversely affects its efficiency.
Further, the boiling point of liquid helium is as low as about 4K. So, the liquid helium container must have a special adiabatic mechanism. Specifically, the special adiabatic structure must be provided, wherein liquid nitrogen is circulated around the container with a vacuum adiabatic layer interposed therebetween; a vacuum portion is provided at the outer periphery thereof; and, in addition, a radiation shield is provided so that it covers the entire container. Therefore, the above-described superconducting motor has a complicated structure and results in a large-sized apparatus.
In addition, since liquid helium is expensive, the operating cost of the superconducting motor is increased.
Furthermore, since the conventional superconducting motor as de scribed above has a complex structure, it is difficult to take apart and put together when maintenance and check is needed, and therefore, it is poor in operation.